1. Field of the Invention
The present invention relates to a combustion chamber structure in engines and more particularly to a combustion chamber structure in engines, in which a gas fuel such as natural gas is supplied into a pre-combustion chamber, air drawn in is supplied into a main combustion chamber and compressed and, near the top dead center on the compression stroke, the compressed air and the gas fuel are mixed and ignited for combustion.
2. Description of the Prior Art
In recent years, engines using gas fuels such as natural gas as a main fuel are being developed for use with cogeneration systems and automobiles. The cogeneration systems produces electric energy from the engine power through a generator and at the same time utilize thermal energy of exhaust gas to heat water by a heat exchanger for hot-water supply. The cogeneration systems are expected to find applications as electric power supply systems in hotels, hospitals and offices. When natural gas fuel engines are used on motor vehicles, their advantage is a low combustion temperature which makes production of NOx difficult, contributing to lowering air pollution.
Engines using natural gas as a fuel include those disclosed in, for example, Japanese Patent Laid-Open No. 156911/1979, 6358/1988 and 232119/1989, Japanese Utility Model Publication No. 41068/1991 and Japanese Patent Laid-Open No. 158448/1995.
The natural gas engine of Japanese Patent Laid-Open No. 158448/1995, for example, has a pre-combustion chamber formed in the cylinder head and a main combustion chamber formed in the cylinder, both communicating with each other through a communication port, with a control valve installed in the communication port. The pre-combustion chamber is connected with a gas chamber through a throttled portion in the cylinder head. The gas chamber is formed with a gas introducing port to supply the natural gas through a gas passage into the precombustion chamber. A gas introducing valve is provided at the gas introducing port formed in the gas chamber and opens when the communication port is closed by the control valve, which opens the communication port near the end of the compression stroke. With the gas introducing valve open, the natural gas is supplied into the gas chamber and stays in the gas chamber and near the throttled portion.
Gas engines in general have conventionally employed a system, because of its simplicity, in which a gas fuel such as natural gas is drawn in from an intake valve, compressed and ignited in the same way as at gasoline fuel. Such gas engines are prone to knocking, which prevents the compression ratio from being increased, so that their theoretical thermal efficiency is not necessarily high.
Because of a poor ignition performance of the gas engine when the fuel is compressed and ignited, it was conceived to form the combustion chamber in a heat insulating structure and perform diesel-combustion to improve the thermal efficiency. As a means to improve the thermal efficiency, the gas engine employs the heat insulating structure for the combustion chamber to convert an exhaust gas energy into a driving power.
Because the gas engine uses a gas such as natural gas as a fuel, however, when a mixture of gas and air is drawn in on the intake stroke and then compressed in a combustion chamber made in the heat insulating structure, the temperature of the mixture highly compressed inside the cylinder becomes abnormally high, making a self-ignition phenomenon or knocking likely to occur. It is known that the thermal efficiency of engines theoretically decreases with a decrease in compression ratio.
Further, because the heat insulating type engine recovers the exhaust gas energy, when the mixture of gas and air is drawn in on the intake stroke and then highly compressed, the temperature inside the cylinder becomes excessively high rendering the self-ignition phenomenon or knocking likely to take place.
The engine with a combustion chamber of a heat insulating structure, because of an abnormally high wall surface temperature of the combustion chamber, has an increased risk of self-ignition, a phenomenon in which the fuel supplied into the combustion chamber becomes ignited before an intended firing timing. Thus, in a heat insulating engine which is set to a high compression ratio, when the air drawn from the intake valve and the fuel gas supplied from the fuel passage are mixed and compressed to a high pressure, the self-ignition will occur, initiating combustion long before the top dead center, causing knocking and making the engine inoperable.
Although the heat efficiency of the engine that burns a gaseous fuel such as natural gas in a diesel cycle can be increased by compressing the natural gas to a high pressure before injecting it into the combustion chamber, this method has disadvantages, which include the fact that compressing the gas such as natural gas to 25-30 MPa entails increased equipment cost and that injecting the gas under high pressure increases NOx emissions, offsetting the economical advantage of mileage improvement by diesel combustion.
Under these circumstances, the conventional gas engines have been made in a configuration in which a combustion chamber is formed into a main combustion chamber and a pre-combustion chamber, both connected to each other through a communication port having a control valve installed therein; whereby a gaseous fuel such as natural gas is supplied into the combustion chamber without compressing the gas fuel to a high pressure; air is supplied into the main combustion chamber in the cylinder during the intake stroke and then compressed on the compression stroke; and the gas fuel is supplied into the pre-combustion chamber during the intake or exhaust stroke and, in the latter half of the compression stroke in which the piston is moving up, the control valve is opened to introduce compressed air from the main combustion chamber into the pre-combustion chamber to mix the air with the gas fuel, after which the mixture is ignited for diesel combustion, thereby increasing the heat efficiency.
In the above gas engine, where the control valve is operated in the second half of the compression stroke to open the communication port connecting the main combustion chamber and the pre-combustion chamber, the pressure of the air compressed in the main combustion chamber during a period up to the opening by the control valve of the communication port rises excessively as the compression stroke advances past its half point toward its end, with the result that increased friction and heat of the compressed air cause an increase in heat loss. The compressed air in the main combustion chamber flows into the pre-combustion chamber at high speed by the pressure difference between the main combustion chamber and the pre-combustion chamber. Although the compressed air pressure in the main combustion chamber is raised to a high level, the communication between the main combustion chamber and the pre-combustion chamber reduces the compressed air pressure, consequently reducing the work of the compressed air by the lower pressure of the gas, deteriorating the mileage to that extent.
In a gas engine with the compression ratio set to 15 to 17, as the piston nears the top dead center with the pre-combustion chamber and the main combustion chamber isolated from each other by the control valve closing the communication port, the compressed air in the main combustion chamber reaches a high pressure 1.7-2 times that of an engine without the control valve. When in this state the communication port is opened, not only does the pressure in the main combustion chamber decrease but also the friction and heat applied to the piston by the high compressed air pressure constitute a negative work, deteriorating the mileage.
Thus, with the gas engine, to limit the pressure of air compressed in the main combustion chamber to not so high a level on the compression stroke and thereby prevent it from being consumed as friction and heat, a part of the compressed air pressure is stored as a spring force, which during the power stroke can be returned to the working fluid that performs a work on the piston. This is considered to improve the mileage.